1. Field of the Invention
The present invention relates to a unit type strain gauge which can be mounted very simply on an object, in which strains are to be measured and easily manufactured in a dust proof and water proof construction.
2. Description of the Prior Art
In the use of the strain gauge such as for the measurement of dynamic strain in railway lines or for the measurement of vehicle weight by mounting the gauge on the shaft of the vehicle, a plurality of strain gauges are used by mounting them on metal parts of the object, in which strains are to be measured. Generally such work is carried out outdoors so that an easy and laborsaving form of mounting is required. Furthermore, for obtaining high accuracy of measurement, rigid mounting of the gauge element, dust proof and water proof characteristics are required.
There is a prior proposal of the present applicant concerning the protection of strain gauges, (Japanese Utility Model No. 1,043,254), in which a protector is used to protect a strain gauge mounting portion on an outdoor structure by providing a dust proof and water proof protecting structure over a strain gauge element mounted on an object. This protector has the drawback that considerable time is required for mounting it because when applying the protector, the strain gauge element is first mounted on an outdoor structure, then lead wires are connected between the element and a connecting cord terminal and finally the protector cover is mounted to establish a water proof construction. The mounting of the strain gauge element on an outdoor object using this known protector is especially difficult during rain.
In more detail, the mounting of the strain gauge element when using said known protector required the following working steps.
A. grinding a surface of the object under test for removing rust and stain. PA1 B. cleaning up the surface by means of a cleaning agent such as toluene. PA1 C. affixing the strain gauge element by using an adhesive. PA1 D. pressing the strain gauge element by means of a pressure applying rubber plate and a pressing metal plate. PA1 E. removing the pressing metal plate and the pressure applying rubber plate after hardening of the adhesive. PA1 F. applying a coating agent to the strain gauge element. PA1 G. mounting a packing member attached to a connecting cable. PA1 H. providing electric connections between the conductors of the connecting cable and the terminals of the strain gauge element. PA1 I. applying an insulative protecting compound to the strain gauge element surrounded by the packing member. PA1 J. placing the protector cover on the compound and pressing it until hardening of the compound.
The mounting of a strain gauge element using the protector cover required several hour's work for the completion of the above steps. During rain, the work is difficult and may require other additional preparations.
There is another proposal in this art that has been disclosed by a British company Rotax Limited for a converter used for measuring the weight of a vehicle. FIGS. 1 to 3 show the construction of a converter made by Rotax Limited.
In the exploded view shown in FIG. 1, two stud bolts 102 and 102' are fixed to a surface 101 of an object in which strains are to be measured by means of welding as shown by 103. A sensing element 105 is mounted on the stud bolts 102 and 102' by means of mounting holes 106 by inserting the bolts in the holes by using two O rings 104. The sensing element 105 is provided with a strain gauge element 107. The strain gauge element 197 is electrically connected to cable conductors 109 at the terminal 108 and is coupled to an external measuring equipment (not shown) through a cable 110. A protecting case 111 is provided to surround the element 105 and an upper cover 112 is mounted on it via a packing 113 and secured by means of four screws 114.
FIGS. 2 and 3 show in more detail the sensing element 105. The body of the sensing element 105 is made in a "cranked" shaped and on both surfaces of the middle portion the gauge elements 107 are adhered and connected to a terminal 108 by means of a gauge lead wire 115. The sensing element 105 is rigidly fixed by means of nuts 116 against the stud bolts 102 and 102'. In this structure, a strain on the surface of the object 101 is conveyed to the sensing element 105 through the stud bolts 102 and 102'. Accordingly, the spacing of the two stud bolts should be made to coincide precisely with that of the mounting holes 106. In practice, this accuracy should be of the order of .+-.5/100 mm (.+-.0.002 inch). Therefore, for this operation expensive jigs and tools and high quality welding are required. For this reason the use of the device is quite limited. Furthermore, in this construction a number of constituent elements are required and hence the device becomes expensive. In addition, since the sensing element 105 has a certain amount of mass and then has an inherent resonant frequency which reduces the frequency response of the element and it does not respond to a sudden shock. Also the case 11 has a breathing effect and hence the structure does not provide complete protection of the gauge against moisture.